1. Field of the Invention
This invention relates to a method of decreasing a resistivity of ZnSe crystal grown by a chemical vapor transport (CVT) method using iodine as a transport agent, comprising subjecting the ZnSe crystal to a heat treatment in a Zn vapor atmosphere.
2. Description of the Prior Art
ZnSe crystal has a high resistivity even if containing iodine to be n-type dopant due to self-compensation by Zn vacancies unless the ZnSe crystal is subjected to a heat treatment after the growth. Thus, this crystal can be caused to have low resistivity by activating iodine through a heat treatment in Zn vapor at a high temperature. In this method, however, if a cooling rate after the heat treatment is small, a vapor pressure of Zn applied is decreased due to lowering of the temperature and Zn holoes are again increased to increase the resistivity. Thus, a rapidly cooling method has generally been employed after the heat treatment.
For example, it has been reported in "J. of Electronic Materials", Vol. 8, No. 5 (1979), p 619-633 that ZnSe crystal grown by a chemical vapor transport method using iodine as a transport agent and Zn are arranged in two quartz ampoules, one of which is heat treated at 800.degree. C. for 13 hours and another of which is heat treated at 830.degree. C. for 24 hours, and both of them are then charged in an oil at 25.degree. C. and rapidly cooled to obtain ZnSe crystals each having a specific resistivity of 0.12 .OMEGA.cm and 0.08 .OMEGA.cm.
When the ZnSe crystal after the heat treatment is rapidly cooled in an oil, however, crystallinity thereof is deteriorated, for example, dislocations are increased and ordinarily, cracks occur in the crystal. This is probably due to that the temperature of the crystal is rapidly lowered, the temperature gradient in the crystal is increased and thermal stress is generated to deteriorate the crystallinity.
The inventors have found that when the ZnSe crystal after the heat treatment is gradually cooled at a cooling rate of 1.degree. C./min, cracks can be prevented, but precipitates often occur in the crystal so that the ZnSe crystal having predetermined crystallinity cannot be obtained.